The present invention relates in general to the measurement of electrical properties of a material, and more particularly to a method and apparatus for holding a specimen of a material to be tested in good electrical contact with the conductors of a coaxial cable in an electrical property measurement system.
With the increasing use of electromagnetic devices of all types in modern times, there is an accompanying increase of electromagnetic energy which is radiated from these devices into the environment. Depending upon its frequency and power level, this radiation is generally undesirable since it can adversely effect the operation of electronic components and electromagnetic devices other than that from which it is radiated. Therefore, it has become necessary to shield many electrical, electromagnetic and electronic devices, such as computers, computer terminals and microprocessors, for example, to protect them from incoming radiation and to prevent outgoing radiation.
Various means have been used to shield electrical and electronic devices by reducing the energy level of incoming and outgoing radiation. Metallic enclosures have proven to be the most effective device for shielding such devices from electromagnetic radiation since almost all types of metallic enclosures are thick enough to provide adequate shielding by both reflection and absorption of incident electromagnetic energy. However, the use of plastic enclosures has become more widespread in recent years due to their lower cost and weight as well as the ease with which they can be fabricated. Plastic materials do not provide electromagnetic shielding in their usual form and therefore must be modified to provide such an effect.
One conventional method for rendering a plastic enclosure effective to shield electromagnetic radiation is to apply a metallic coating to the plastic, thereby enabling the plastic enclosure to function in the same manner as a metallic enclosure. Another conventional method for rendering a plastic enclosure effective to shield electromagnetic radiation is through the use of conductive composites. The conductive composites are made by incorporating a conductive filler in a resin. The conductive filler can be carbon particles or metallic particles such as aluminum flake or aluminum fibers, for example. Both coated plastics and conductive composites have adequate electromagnetic shielding capabilities to enable them to be utilized in a variety of applications.
In the manufacture of plastics which have been treated to give them electromagnetic shielding capabilities, it is necessary to measure the shielding effectiveness of the material. Shielding effectiveness (SE) is generally defined as the ratio of the power which is incident on a material to the power which is transmitted through the material and is usually expressed in units of decibels, i.e., EQU SE=10 log (P.sub.i /P.sub.t) in dB.
In the past, shielding effectiveness has been measured by placing a flat panel of the material to be tested across an opening in a large room which is totally enclosed by steel walls so that no electromagnetic radiation can escape other than through the panel. A transmitter is placed in the room and a receiver is located outside the room and the power level of the signal received at the receiver is measured without the panel across the opening in the room and subsequently with the panel in place across the opening. The ratio of the two measured signals provides an indication of the shielding effectiveness of the panel of material over a relatively broad range of frequencies. If the measurement of shielding effectiveness is limited by space or cost considerations, essentially the same procedure can be carried out using a small box in place of the large room.
These prior art methods have a number of limitations which seriously effect the accuracy of the data obtained during the measurement. In addition to the cost of constructing a large shielded room and the use of large material specimens, these limitations include the effects on the measurement of the resonance of the room at critical wave lengths, the resonance of the aperture in the room and the frequency response characteristics of the transmitter and receiver antennas. It has been found that these limitations are substantial enough to produce different sets of data from different measurement facilities even though the same material specimen is being measured at each facility.
More recently, a method of measuring the shielding effectiveness of material specimens with the use of a coaxial conductor has been developed to overcome the limitations associated with the previously discussed method of measuring shielding effectiveness. In this method, the material to be tested is placed in electrical contact with the conductors of a coaxial cable. The shielding effectiveness is determined by a standard substitution method conventionally used to measure insertion loss of various components at different radio frequencies, wherein the power level of a signal transmitted from one end of the coaxial cable and received at the other end is first measured without the material in contact with the conductors of the cable and then with the material in place in the measurement system. The amount of attentuation of the transmitted signal introduced by the material specimen is detected and provides an indication of the shielding effectiveness of the material.
This coaxial line method of measurement has several advantages over the prior method discussed previously. Among other features, it is capable of functioning effectively over a broad band of frequencies and it operates in the transverse electromagnetic mode, i.e., the same mode at which energy is propagated in free space, to thereby render an effective indication of the shielding effectiveness of the tested material in a practical application. The electromagnetic signal used in the measurement of the effectiveness is totally enclosed within the coaxial line and therefore leakage is not a significant factor in the measured data. Resonances which may be due to room size, antenna characteristics and the reflections of surrounding structures are eliminated. In addition, the coaxial cable enables relatively small specimens of the material to be used in the measurement system.
One problem associated with the coaxial line method of measuring shielding effectiveness is the difficulty of attaining good electrical contact between the material specimen and the inner and outer conductors of the coaxial line. In order to achieve good electrical contact, it is necessary to have good physical contact. If good physical contact is not obtained, there will be a gap between the material specimen and the walls of the coaxial transmission line, which will introduce a capacitive reactance into the line in series with the predominantly resistive load of the shielding material. This capacitive reactance can seriously effect the accuracy of data which is obtained during the shielding effectiveness measurement. It is not economically feasible to machine a specimen of material to the precise dimensions necessary to achieve good electrical and physical contact between the material and the conductors of a standard coaxial cable.
One device for use in the measurement of shielding effectiveness in a coaxial line is illustrated in U.S. Pat. No. 2,747,160, issued to Byrd. The Byrd patent discloses a measuring device for placing wire insulation, such as that found on ignition wires, in contact with the conductors of a coaxial cable to enable the shielding effectiveness of the wire insulation to be measured. The device disclosed in the patent can only be used to measure the effectiveness of wire insulation or other tubular materials, and is not suitable for use in measuring the shielding effectiveness of a specimen from a sheet of material. In addition, the dimensions of the wire insulation and the device for holding the insulation must be within very small tolerance ranges in order to achieve good electrical contact between the coaxial cable and the specimen of insulation, resulting in a substantial cost factor in the measurement of shielding effectiveness.
It is therefore an object of the present invention to provide a novel method and apparatus for holding a specimen of material in good electrical contact with the conductors of a coaxial cable.
It is another object of the present invention to increase the applicability of the coaxial line method of measuring shielding effectiveness by providing a novel specimen holder which can accommodate planar specimens of a material to be tested.
It is a further object of the present invention to provide a novel method and apparatus for testing the shielding effectiveness of material specimens.